Похоже, что в python3 объект Header
необходим для кодирования Subject
как "utf-8":
>>> from email.mime.text import MIMEText
>>> from email.header import Header
>>> s = 'ação'
>>> m = MIMEText(s.encode('utf-8'), 'plain', 'utf-8')
>>> m['Subject'] = Header(s, 'utf-8')
>>> m.as_string()
'Content-Type: text/plain; charset="utf-8"\nMIME-Version: 1.0\nContent-Transfer-Encoding: base64\nSubject: =?utf-8?b?YcOnw6Nv?=\n\nYcOnw6Nv\n'
Таким образом, оригинальный сценарий стал бы:
servidor = smtplib.SMTP()
servidor.connect(HOST, PORT)
servidor.login(user, usenha)
assunto = str(self.lineEdit.text())
para = str(globe_email)
texto = str(self.textEdit.toPlainText())
corpo = MIMEText(texto.encode('utf-8'), 'plain', 'utf-8')
corpo['From'] = user
corpo['To'] = para
corpo['Subject'] = Header(assunto, 'utf-8')
servidor.sendmail(user, [para], corpo.as_string())